1. Field of the Invention
This invention relates to a disposable syringe for once-only use, (i.e., is shaped such that it is impossible to move the plunger within the syringe cylinder after said plunger has been once pushed to the bottom of the syringe).
2. Discussion of the Background
There is known to be an increasingly urgent requirement for syringes which can be used once only, (i.e., which are unable to draw in new liquid once the original liquid initially drawn into the syringe has been injected).
This is to prevent the repeated use of syringes and the resultant spread of certain infective illnesses which currently afflict humanity.
The problem has been studied throughout the world and many solutions have been proposed. Some of these are very complex and costly, such as those described in the patents U.S. Pat. No. 4,699,614 and GBP-A-2,205,750.
The patent applications PCT WO 88/10127, GB-A-2,015,883 and EP-A-0,325,886 describe syringes in which the plunger and its operating shaft are only weakly linked together, i.e., can disengage and various expedients are provided to ensure that when the plunger has been pushed to the bottom of the cylinder by the action of the shaft, the shaft disengages from the plunger when the shaft is pulled. The plunger thus remains retained at the bottom of the syringe whereas the shaft can be withdrawn from the cylinder but without any possibility of drawing new liquid into the syringe. To retain the plunger more securely in the cylinder, shaped annular ribs or teeth are provided projecting from the inner surface of the cylinder to oppose any movement of the plunger in the direction of its withdrawal from the syringe needle. Syringes constructed in this manner are very costly, both because of the difficulty of making the weak linkage (but in any event strong enough to allow the initial in-drawing of liquid for the first filling of the syringe) between the plunger and shaft, and because of the problems involved in forming internal elements rigid with the cylinder and able to oppose or block the drawing movement of the plunger.
The British and French patent applications GBP-A-2,117,249 and FR-A-2,181,580 describe syringes in which the plunger is provided with a flexible arched flange the free ends of which slide in contact with the inner surface of the cylinder. The concavity of the flange faces the open free end of the cylinder so that when the plunger is pushed forwards (towards the needle), the flange bends and allows the plunger to slide freely. If an attempt is then made to retract the plunger (by withdrawing it from the needle in order to draw liquid into the syringe), the free edges of the flange rub fiercely against the inner surface of the cylinder, to jam against it and prevent withdrawal of the plunger. It can be seen that this type of system does not even allow the initial filling liquid to be drawn into the syringe, which has to be pre-loaded, (i.e., already filled with the liquid to be injected before the plunger is inserted into the syringe cylinder).
The U.S. Pat. No. 3,934,586 describes a syringe the cylinder of which comprises an internal groove or recess against which a tooth projecting from the syringe and obtained by manually breaking a breakable tab formed integrally with the shaft rests (to thus prevent withdrawal of the shaft). This syringe is also of complex structure and must be pre-filled with the liquid to be injected.
French patent application FR-A-2,622,804 describes a syringe, in the cylinder of which, in proximity to that end at which the hypodermic needle is mounted, there is provided a continuous annular recess into which a shaped elastically flexible portion of the shaft which projects radially outwards becomes inserted in a non-removably manner. To aid in the retention of the shaft against the syringe cylinder when the plunger has been pushed to the bottom of the cylinder, an annular groove can be provided in proximity to the free end of the cylinder, into which the shaft head, in the form of a flat thin disc of shape and dimensions complementary to those of said annular groove, penetrates by snap-insertion. In this manner, when the plunger is pushed to the bottom of the cylinder, the plunger head penetrates totally into the cylinder and can no longer be gripped.
The syringe described in French patent application FR-A-2,622,804 is of truly complicated structure and therefore costly (i.e., due to the presence of the recesses in the cylinder and the corresponding deformable shaped portion of the shaft), whereas the presence of the groove provided in proximity to the free end of the cylinder for snap-housing the disc-shaped head of the shaft is superfluous because engagement between said deformable shaped portion of the shaft and the relative cylinder recesses is already provided for, and it would also be superfluous even if this latter engagement did not exist, because the flat head could be gripped with the finger (in order to withdraw the plunger) up to the point at which the head has been completely pushed into its seat in the cylinder, which means that the syringe would be reusable many times by simply taking care not to push the plunger completely to the bottom of the cylinder. Finally, it can be seen that to push the shaft head right down until it penetrates into the respective locking groove in the cylinder would be very difficult in the case of small syringes (such as insulin syringes), because the finger used to press against the plunger head would rest against the free end of the cylinder, making it impossible to further push the shaft head, as this would be of very small diameter and thickness.
British patent application GB-A-2,197,792 describes a syringe the cylinder of which comprises at its free end a cylindrical sleeve into which the shaft head penetrates completely when the plunger is pushed to the bottom of the cylinder, so that under these conditions the head can no longer be gripped with the fingers to reuse the syringe. Engagement elements can be provided to lock the shaft head in said cylindrical sleeve. The drawback of this syringe is that the shaft head is unable to be gripped with the fingers to reuse the syringe only if this head is pushed completely into the space defined by the cylindrical sleeve, which is difficult to do in the case of a small-dimension syringe, such as syringes used for injecting insulin or for intravenous injections. If the shaft head projects beyond the free edge of the cylindrical sleeve, it can be gripped with the fingers and further liquid can be drawn into the syringe.